Rolling mills for forming seamless tubing have taken various forms. These mills differ generally in the arrangement and design of the rolling mill for the pre-rolling of hollow blocks to pipes arranged between the rotary piercing mill (or punch press) and the stretch-reducing rolling mill.
In a classic design of a pipe rolling mill, the mill is equipped with a multi-stand continuous rolling mill to which the hollow blocks are delivered as they come from the rotary piercing mill or the punch press.
If the round billets are provided with a bore in a rotary piercing mill to form the hollow blocks, then these hollow blocks must be delivered to the continuous rolling mill by a cross coneyor. The rotary piercing mill usually works with a mandrel arranged on the exit side of the mill; the round billets are formed into hollow blocks by rotating the billet about its longitudinal axis on the mandrel. The rotary piercing mill does not permit immediate introduction of the hollow block into the continuous rolling mill with a mandrel arranged on the outlet end, because the mandrel has to be removed from the block after the rolling operation. Even in the case where the mandrel does not have to be removed at the outlet side, the hollow block would not be able to move directly into the continuous rolling mill, because it rotates about its axis during the shaping operation in the rotating piercing mill; such rotation is not desirable in the continuous rolling mill.
The deformation of the hollow block within the continuous rolling mill takes place on a mandrel which is introduced into the hollow block in front of the continuous rolling mill, the mandrel moving along with the block in the direction of rolling during the operation of rolling the seamless pipes. For this reason, the mandrel must have a length corresponding at least to the length of the seamless pipe.
In a rolling mill equipped with a continuous rolling mill, it is also necessary to provide a mandrel-removal device on the exit side of the continuous rolling mill. With the help of this device, the mandrel is removed after the seamless pipe leaves the continuous rolling mill, the mandrel being carried back to the entrance side of the continuous rolling mill for re-use.
The presence of the mandrel-removal device makes it impossible to arrange the continuous rolling mill and the stretch-reducing rolling mill which follows it along one rolling line. For this same reason the seamless pipes have to be moved on a cross conveyor located after the continuous rolling mill to bring them to the entrance to the stretch-reducing rolling mill. Because of the relatively long path taken by the pipes in passing through the continuous rolling mill and the following mandrel-removal device, the seamless pipe cools off to an approximate temperature of 650.degree. C., so that it has to be reheated before introducing it to the stretch-reducing rolling mill. For this purpose, a reheat furnace is located after the cross conveyor in these known rolling mill plants to heat the seamless pipes before they can be introduced into the stretch-reducing rolling mill. It can be clearly seen that a rolling mill of the type described is very expensive, because it requires, in addition to the different rolling mills, expensive auxiliaries, so that the plant can only be housed in long, wide buildings. In addition, the weight of the round billets that can be used for making the pipe is limited, because the seamless pipes (as intermediate products) cannot exceed a maximum length of 30 meters, based on the design of the auxiliary devices.
A completely continuous fabrication of seamless tubing with such a rolling mill plant, as described for example in the "Klepzig Professional Journal" 81st year 1973, pages 218-221, is not possible.
To reduce the cost of the plant, a suggestion was made in the periodical "Stahl & Eisen" 93rd year, 1973, page 1026 in the right hand column, to locate a planetary rotary piercing mill in place of the multi-stand continuous rolling mill. In general, there would be no change in the general design of the rolling mill, because it is evident that the rolling of seamless pipes must also be made on a following mandrel; consequently, the mandrel-removal device, the added cross conveyor, and the reheat furnace located ahead of the stretch-reducing rolling mill must still be present.
Rolling mills for the fabrication of seamless pipes made from round billets are also known in which a so-called "Assel" rolling mill is provided with the rotary piercing mill for the fabrication of hollow blocks. On the "Assel" rolling mill, the re-shaping of the hollow blocks into seamless pipes is also performed on a mandrel, but located at the inlet side. During the rolling operation, the stock rotates about its longitudinal axis and the mandrel partakes of this rotation.
In order to fabricate seamless pipes on an Assel rolling mill, a mandrel-removal device is not required, but in this case the seamless pipe also cannot run directly into the stretch-reducing rolling mill, mainly because rotational movement of the pipe is undesirable in the stretch-reducing rolling mill. Therefore, the Assel rolling mill must also be used with a cross conveyor and a reheat furnace, so that the cost in such case is still quite high.
In the Assel rolling mill, the necessity of supporting the mandrel at the entrance side creates considerable difficulties, because it rotates with the rolling stock and, therefore, must be supported by expensive bearings capable of absorbing axial forces.
To complete the discussion of the prior art, it should be mentioned that there is a so-called "reciprocating" rolling process mill available for the fabrication of seamless pipes. The type of operation is somewhat between that of the two previously-described rolling mill types, because the rolling stock does not rotate continuously but is turned intermittently. Also, pipes coming from the reciprocating rolling mill cannot go directly into the stretch-reducing rolling mill arranged after it.
It is, therefore, an outstanding object of the invention to do away with the disadvantages of all the previously-described rolling mills for the fabrication of seamless pipes from round billets. Therefore, it is the purpose of this invention to create a rolling mill of the previously-described type for fabrication of seamless pipes from round billets in which the hollow blocks which have been formed in a rotary piercing mill (or on a punch press) are rolled continuously and with one heating into the finished product. At the same time, the weight of the individual round billets can be increased, so that the production of this rolling mill is improved correspondingly.
A further purpose of this invention is to simplify even more the operating procedure of a pipe rolling mill plant by replacing the continuous rolling mill for rolling of seamless pipe with a planetary rotary piercing mill; therefore, it is a purpose of this invention to equip the planetary rotary piercing mill for continuous rolling of pipe billets to seamless pipes in such a way that at least the commonly-used mandrel pullout device is eliminated.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.